>> class Person
>>   def age
>>     self.calculate_age
>>   end
>>
?>   def calculate_age
>>     'age from person'
>>   end
>> end
=> nil
>>
?> class Joe < Person
>>   def calculate_age
>>     'age of Joe'
>>   end
>> end
=> nil
>> j = Joe.new
=> #<Joe:0x257a9c>
>> j.age
=> "age of Joe"
>> p = Person.new
=> #<Person:0x252b14>
>> p.calculate_age
=> "age from person"
>>